1. Field of the Invention
This invention pertains to novel water-compatible, vinyl ester resins curable with actinic or ionizing radiation inducement and their use in coating substrates. More particularly, this invention pertains to such compositions derived from epoxy resins.
2. Prior Art
It is well known that many vinyl ester resins are polymerizable by radiation inducement. This is illustrated by the disclosure in U.S. Pat. Nos. 3,560,237 (RE 27,656); 3,661,576; 3,673,140 and British Pat. No. 1,375,177. The disclosures of U.S. Pat. Nos. 3,560,237 (RE 27,656) and 3,661,576 are particularly relevant to the instant invention in that they pertain to vinyl ester resins derived from resinous epoxides (i.e., epoxy resins). Many of these vinyl ester resins from resinous epoxides are commercially available and possess excellent physical and chemical properties and are particularly useful as protective coatings for a variety of substrates. In this utility, the vinyl ester resins were normally dissolved in an organic solvent or a reactive diluent, applied by any one of several conventional techniques (e.g., spraying, dipping, etc.) to a substrate and then cured with actinic radiation.
It is also known that certain sulfonium salts can be used as accelerators for the photopolymerization of acid polymerizable and/or acid-curable materials (including epoxy resins per se, Netherland Patent Application No. 75.10320). This patent also teaches that the sulfonium salts must be soluble in the resinous component and states that the ability of a sulfonium salt to accelerate the cure of any particular material depends upon the ability of the corresponding acid, i.e., the acid containing the same anion as the sulfonium salt, to cure that material. These photocurable compositions were similarly dissolved in an organic solvent and applied to various substrates by conventional techniques.
The necessity of using an organic solvent is a commercial impediment for using the photocurable compositions set forth above. Organic solvents must be recovered and recycled or disposed of for safety, environmental and/or economic reasons. The recovery in many instances is difficult and/or costly. These problems can be reduced if the organic solvent reacts into the coating. Considerable research has been directed toward the use of vinyl monomers and low viscosity vinyl resins are reactive diluents. However, many of the useful reactive diluents, e.g., 2-hydroxyethyl acrylate, are toxic and represent considerable health and environmental problems.
Presently available radiation-curable systems use a reactive diluent such as an acrylic monomer to reduce the viscosity to the level required for application. Many of these diluents suffer from excessive toxicity, volatility or odor. In addition, since the diluent is incorporated into the final product, the amount and kind of diluent will affect the properties of the end coating. With conventional solvent systems, additional solvent can be added to adjust viscosity without materially affecting the properties of the final coating. However, the presence of these solvents poses a toxicity problem.
The commercial problems enumerated above have caused many potential customers to use alternate systems having different mechanisms of cure and/or different polymer structure, e.g., latexes.
Many resinous systems have been rendered water-soluble or water-dispersible (oil-in-water dispersions) by attaching various onium (e.g., sulfonium, phosphonium, ammonium, etc.) groups to the backbone of the resin or by adding an onium surfactant to the resin as a dispersing vehicle. Many of these onium compounds are electroreducible, particularly the sulfonium and isothiuronium compounds, and have been used in cathodic electrodeposition processes. An exhaustive documentation of this is not required; however, reference is made to U.S. Pat. Nos. 3,793,278; 3,936,405; 3,937,679; 3,959,106 and 3,894,922 which represent a series of cases in which certain onium-modified epoxy resins were alleged to be useful as electrodepositable compositions. The onium-modified epoxy resins were prepared by reacting an epoxy resin with a tertiary phosphine, tertiary amine or sulfide in the presence of an acid. The acid used in this series of experiments had dissociation constants greater than 1.times.10.sup.-5 and included both organic and inorganic acids. Alkenoic acids, while meeting the dissociation constant criterion in many instances, were not named or used in any of these particular references.
Harris et al. (U.S. Pat. No. 4,020,030) prepared water-soluble or water-dispersible (oil-in-water) sulfonium-modified epoxy resins having superior coating properties and lower toxicity to those set forth above by using epoxy resins having an epoxy equivalent weight greater than about 500 and/or by converting greater than about 70 percent of the epoxy groups in the epoxy resins by reaction with a sulfide and an acid. These sulfonium-modified epoxy resins were used as coatings on various substrates and were applied by conventional techniques (e.g., spraying, dipping, etc.) as opposed to electrodeposition. The disclosure of Harris et al, is incorporated herewith by reference. Harris et al. found that acids having a dissociation constant greater than 1.times.10.sup.-5 were suitable in their preparation of sulfonium-modified epoxy resins and they explicitly state that alkenoic acids (e.g., acrylic acid, methacrylic acid, etc.) are satisfactory in their process. However, these acids were converted to sulfonium alkenoates; vinyl ester resins were not considered. Harris et al. do not indicate that any of their compounds are (or would be) photocurable.